How to test surge arrester?

Testing a surge arrester means checking, through inspection and measurement, whether it can still hold back transient overvoltages. Technicians usually combine a close visual check, leakage current readings, thermal imaging and, where the equipment can be isolated, off-line electrical tests. The combination matters because a clean housing says little about the zinc-oxide blocks inside, which can age or absorb moisture unseen. This guide covers each method, what it can and cannot reveal, and how results are read.

What Does an Arrester Actually Do?

An arrester (often written “surge arrestor” in product listings) sits between a live conductor and earth. Its job is to cut off the voltage spikes caused by lightning strikes and switching operations before they reach transformers, cables and switchgear.

Modern gapless designs contain a stack of metal-oxide varistor (MOV) blocks. At normal operating voltage, the blocks act almost like an insulator and pass only a tiny leakage current. When a spike arrives, their resistance collapses, the surge current runs to earth, and the voltage across the protected equipment stays limited. Once the event passes, the blocks return to their high-resistance state. If that behaviour degrades, protection degrades with it.

Why Testing Matters?

Arresters do their work quietly, and a failure often becomes obvious only when the equipment behind them is damaged. Several things wear them down:

• Ageing of the MOV blocks over years of service
• Moisture ingress through a damaged seal
• Thermal stress from sustained heating
• Electrical overstress from severe or repeated surges
• Housing damage, including cracks, tracking and heavy contamination

Much of this happens inside the unit, so an arrester can look normal from the ground while its leakage current is creeping up. Periodic condition assessment lets you plan a replacement instead of reacting to a failure.

How to Test a Surge Arrester?

There is no single procedure for every unit. The methods chosen depend on the arrester type, voltage class, manufacturer guidance and whether the equipment can be tested in service or must be disconnected. Thermal imaging and leakage current measurement are online methods. Off-line testing needs isolation.

1. Visual Inspection

This is the starting point. Technicians look for cracked or chipped housings, pollution build-up, tracking marks, corroded fittings, loose or discoloured connections, and the state of the disconnector or fault indicator. It is quick and worth doing often, but it cannot confirm that the blocks inside are healthy.

2. Leakage Current Measurement

Even under normal voltage, a metal-oxide arrester passes a small current to earth. Degradation tends to change that current, which is why it is monitored.

Total leakage current has two parts. The capacitive part is larger and depends mostly on the arrester’s construction and surroundings. The resistive part is much smaller but reflects the condition of the blocks far more directly. A change in the total figure alone can therefore be misleading. Readings need to be judged against reference conditions and the manufacturer’s information, because no universal value separates a good unit from a bad one.

3. Resistive Leakage Current Testing

Since the resistive component says the most about the blocks, diagnostic instruments try to isolate it, either by separating it from the capacitive component or by compensating for it. One reading rarely tells the whole story. Repeat measurements under similar conditions, and comparison with the other phases, make a slow upward drift much easier to spot. Voltage, temperature and surface pollution all influence the result, so record them alongside it.

4. Thermal Imaging

An infrared camera can scan an energized arrester from a safe distance and show abnormal heating. The useful comparison is against neighbouring phases or identical units under the same conditions. Ambient temperature, sunlight, wind, load and camera settings all affect the picture, so note them. A hot spot justifies a closer look, but it is an indicator rather than proof of failure, and a normal image does not guarantee a healthy arrester.

5. Surge Counter and Fault Indicator Checks

A surge counter records discharge events. A disconnector or fault indicator is meant to show that the arrester has failed and separated from the system. Both help build a service history. Their limits are real, though: counters can miss events, and a disconnector will not necessarily operate for gradual deterioration.

6. Off-Line Testing

With the arrester disconnected, separate test equipment can take controlled measurements that are not practical in service. This work belongs to qualified personnel, with proper isolation, earthing and clearances, following the manufacturer’s instructions.

What Parameters Are Checked?

No single parameter proves an arrester is good or bad.

How to Know If an Arrester May Be Faulty?

Warning signs include visible cracking, abnormal heating, rising or unusual resistive leakage current, an operated disconnector, moisture ingress, and readings that differ from identical units nearby. Interpretation should weigh manufacturer specifications, earlier results, system voltage, temperature, environment, arrester type and applicable standards. No fixed limit suits every design.

Routine Inspection vs Diagnostic Testing

How Often Should Testing Be Done?

There is no interval that fits every site. Periodic inspection and condition assessment should reflect manufacturer recommendations, the importance of the protected equipment, system voltage, environment, lightning exposure, previous results and site procedures. A worsening trend can justify shorter gaps between checks.

Safety Precautions

Arresters are usually connected to high-voltage systems, so testing carries serious electrical hazards. Only appropriately qualified personnel should do it:

• Follow site safety procedures.
• De-energize and isolate equipment when required.
• Apply lockout/tagout where applicable.
• Verify absence of voltage.
• Use properly rated test equipment.
• Maintain required clearances.
• Follow manufacturer instructions.

Standards and Manufacturer Guidance

IEC 60099-5 gives selection and application recommendations and discusses diagnostic approaches for in-service metal-oxide arresters. It does not set one pass/fail threshold for every design. Confirm which edition applies to your equipment, and read the datasheet for any surge arrestor you are assessing, since manufacturers differ. Other IEC and IEEE standards, local safety rules and site procedures may also apply.

Common Mistakes to Avoid

1. Relying only on visual inspection.
2. Using a single reading with no historical comparison.
3. Ignoring temperature and environmental conditions.
4. Applying generic pass/fail limits instead of manufacturer data.
5. Testing without proper isolation and safety procedures.
6. Dismissing abnormal readings because the arrester looks fine.
7. Treating total leakage current as the only parameter.

Conclusion

A reliable verdict on a surge arrester comes from combining evidence: inspection findings, leakage current trends, thermal images and indicator status, all read against manufacturer data and past readings. Treat every result as an indicator, keep records so trends stay visible, and leave energized or isolated testing to trained people. If you need help with arrestor testing, you can contact Lax Energy Solutions Pvt Limited.

Frequently Asked Questions